1. Field of the Invention
Embodiments of the present invention relate to apparatuses and methods for processing semiconductor substrates. More specifically, embodiments of the present invention relate to apparatuses and methods for resistively heating a thermal processing system.
2. Description of the Related Art
The fabrication of an integrated circuit (IC) on a semiconductor substrate may involve a number of processing steps that are widely varying in scope, nature, or purpose, but which may have in common the fact that they are carried out at an elevated temperature. Examples of IC manufacturing technologies that may involve a heating step include epitaxy, thin film deposition of both dielectric and conducting layers, ion implantation, annealing, junction formation, and the like. Thermal processing may be carried out in a processing chamber having radiant heat sources, such as lamps, RF sources that heat inductively, or resistively heated sources such as heater blocks or susceptors adjacent to a substrate support.
Thermal processing chambers may be resistively heated. A thermal processor of this type may include heating elements connected to a power supply. As a voltage is applied to the heating elements, the resistance of the elements to the flow of electrical current results in a dissipation of power, which provides heat flow to the processing chamber.
A conventional method of resistively heating a processing chamber makes use of a heating element comprising a graphite core coated with a layer of silicon carbide. The silicon carbide is used to seal the graphite core since the graphite may contain impurities. The presence of these impurities, which may be metallic impurities, is undesirable to IC manufacturers since they can find a way onto the semiconductor substrate being processed and interfere with device performance. The silicon carbide coating offers a protective layer that allows a heater comprising a graphite core, with its impurities and contaminants, to be inserted into a reaction chamber.
Conventional heating elements cause a variety of problems. One is that the core and coating materials may have different coefficients of thermal expansion, and as a consequence of the graphite core expanding at a rate different from that of the silicon carbide, the heater may flex or undergo a distortion in shape as the heating element changes temperature.
A second problem may arise as a result of the difference in thicknesses of the two materials comprising the heating element. In some systems, the silicon carbide coating may be only about 0.004 inches thick. Again, due to the difference in thermal expansion coefficients between the two materials, the silicon carbide coating can crack upon heating and cooling of the heating element, exposing the interior of the processing chamber and the substrates being processed to impurities in the graphite. Impurities from the graphite may then diffuse through the cracks in the silicon carbide coating, out of the resistive heating element and into the reaction chamber, causing contamination.
This problem is exacerbated if there is an oxygen environment in the reaction chamber, which may be the case if photoresist is being stripped from a substrate, or if a thermal oxidizing process is being performed in a rapid thermal processing (RTP) chamber. Additionally, oxygen may leak into the chamber through the seals that isolate the chamber from the external environment. Oxygen may diffuse through cracks in the silicon carbide coating to react with the graphite core. The graphite reacts with the oxygen to form carbon monoxide and carbon dioxide gasses (this process is called “ashing”), and the inner core of the heating element may be rapidly eroded. A hot spot results because the resistance of the heater has been dramatically increased at that location where the ashing is taking place. This is a very aggressive reaction and the inner core of the heating element may be consumed in as little as 10 seconds.
A third problem that may be encountered with conventional resistive heating elements is that often the core material is not particularly strong. This is true of graphite. Because of the relative mechanical weakness of a core material such as graphite, the cross-section of a heating element is often large to compensate for its lack of strength. A large size, however, may present other problems. A large heater may have a larger thermal mass as well, which can make the temperature less responsive to changes in power. Delayed temperature changes may result in poor temperature repeatability. Another consequence may be a decrease in the number of substrates that can be processed per unit time (“throughput”) due to delays encountered as the chamber achieves the desired processing temperature.
Conventional heating designs have addressed some of these problems by encasing heating elements within an enclosure, as depicted in FIG. 1. The resistive heater shown generally at 100 in FIG. 1 comprises four heating elements 101, 102, 103, and 104 (that may be part of the same trace winding into and out of the plane of the figure), within an enclosure that includes an upper shield 106 and the lower shield 108. Each of the heating elements has a graphite core 110 and a silicon carbide coating 112. The enclosure comprises a graphite portion 114 and a silicon carbide coating 116. The heating elements may be attached to the enclosure with supports 118. In this example, the thickness of the heating elements and the enclosure parts are about 0.25 inches, and the thickness of the silicon carbide coating is about 0.004 inches. An inert gas such as nitrogen, helium, or argon, may flow through space 122.
Again, the reason for enclosing the heating elements within a shell of SiC-coated graphite is that the SiC coating of the heating element is susceptible to cracking due to the different coefficients of thermal expansion (CTE) of the two materials. If the coating cracks, the graphite core is vulnerable to ashing, especially if the heating element is exposed to an oxidizing environment. Ashing of the graphite can lead to an immediate “burn out” and loss of the heater, as well as a release of impurities into the chamber. By enclosing the heating elements within a shell (or shield), it is possible to flush an inert gas into the space surrounding the elements to purge these impurities out of the chamber. The inert gas also serves to prevent any oxygen in the reaction chamber from gaining access to the graphite core.
Thus, exemplary conventional heaters may be thought of as having three pieces: 1) the heating element comprising silicon carbide coated graphite, 2) an enclosing shell comprising silicon carbide coated graphite, and 3) an inert gas being used to purge the space outside the heating element but inside the shell. Upper and lower shields 106 and 108 also serve to provide a more uniform heating environment by distributing the heat flow from the individual elements. Although the shields are successful in diffusing heat from the elements to make a more uniform output, they may also add to the thermal mass of the heater making the heater less responsive to changes in power. The use of shielding of a conventional heater also may result in large and bulky hardware. An exemplary processing chamber containing conventional heaters 202 and 204, respectively, above and below a susceptor, and side heater 206, is shown generally at 200 in FIG. 2. This figure illustrates the large amount of space taken up within a processing chamber utilizing a conventional resistive heater.
What is desired are apparatuses and methods for resistive heating of semiconductor processing which provide, among other things: a compact, less massive configuration, resistance to an oxygen environment, low potential for contamination or degradation, and more predictable changes to shape during thermal expansion and contraction.